6.1: Describing Chemical Reactions
page 225
Chemists use word equations to describe a wide variety of
chemical reactions, simple and complicated, that occur everyday.
In a chemical reaction, one or more substances change into
different substances. One of the most familiar types of reactions is
a combustion reaction.
Chemists have developed a set of rules that enable people around
the world to communicate and share information about chemical
reactions.
Describing Chemical Reactions with Equations
Word Equations is one way of representing a chemical reaction: it
tells what reacts and what is produced.
Writing Word Equations
Word equations follow a specific format. Reactants are found on
the left side and products are found on the right side. An arrow (→)
is used to give direction, from reactants to products.
All Reactants → All Products
Plus (+) signs are used to identify the reactants being combined
and the products being produced.
Reactant #1 + Reactant #2 → Product #1 + Product #2
Word Equations for some Chemical Reactions
Rusting iron is a slow process. The reactants are iron and oxygen,
and the product is iron (III) oxide.
Word Equations:
iron + oxygen → iron (III) oxide
When copper is placed in a silver nitrate solution, the products
would be silver and copper (II) nitrate.
Word Equation:
copper + silver nitrate → silver + copper nitrate
When zinc is added to hydrochloric acid, hydrogen gas and zinc
chloride are produced.
Word Equation:
zinc + Hydrochloric acid → hydrogen gas + zinc chloride
Chemical reaction can either absorb or release energy.
Word Equation:
Iron + sulfur →
iron (III)sulfide + energy
In this reaction, more energy is released than absorbed. The word
„energy‟ is therefore written on the right side of the equation, with
the products. If energy is absorbed then it is written on the left side,
with the reactants.
Chemical Equations
Chemical reactions can be communicated as a word equation or
chemical equation. The chemical equation provides more detail: it
gives the chemical formulas of the reactants and products, as well
as their state.
Table 1: common state Symbols in Chemical Equations
Word Equation:
energy + copper (II) carbonate → carbon dioxide + copper (II)
oxide
Chemical Equation:
Energy + CuCO3 (s) → CO2 (g) + CuO (s)
Check Your Learning: questions 1 – 9,
pg. 227
Try This Activity: Completing Word Equations
1. aluminum +
→ aluminum oxide
2. zinc + oxygen →
3. aluminum + copper (II) chloride → copper +
4. sodium sulfate + calcium chloride → calcium sulfate +
6.3: Conserving Mass in Chemical Reactions
page 230
Investigation 6.2: ‘Is Mass Gained or Lost During a Chemical
Reaction?’, we see two solutions react to form a solid precipitate.
The mass of the reactants is equal to the mass of the products.
There was no mass gained or lost.
Does this hold true for all reactions or are there reactions that
may gain or lose their mass?
During a combustion reaction, like the burning of wood, it appears
the wood is changed to a pile of ash and mass is lost. Is mass truly
lost or is there something else produced?
During this reaction, gases are used as reactants and formed as
products. To determine whether mass is lost, scientist must be able
to measure the mass of these gases.
After years of experimenting, scientists have determined that mass
is neither gained nor lost in any chemical reaction. This is stated in
the Law of Conservation of Mass.
“In any given chemical reaction, the total mass of the reactants
equals the total mass of the products.”
Scientific Law - is a general statement that sums up the
conclusions of many experiments, or a statement that summarizes
an observed pattern in nature.
The Law of Conservation of Mass
- states that ,in a chemical reaction, the total mass of the
reactants is always equal to the total mass of the products.
Since the mass of the reactant is equal to the mass of the products,
the number and type of atoms in the reactants must equal the
number and types of atoms of the products. (Balanced Reaction)
Experiments have shown that during a chemical reaction, the
atoms in the reactant molecules are rearranged to form products.
A chemical equation must follow the law of conservation of mass,
it must show an equal number of each kind of atom on both sides
of the equation.
Figure 2: there are two (red) oxygen atoms and one (black) carbon atom on either side of
the scale. Similarly, there are two oxygen atoms and one carbon atom on either side of
the arrow in the chemical equation.
i.e., carbon + oxygen → carbon dioxide
C (s) + O2 (g)
→
CO2 (g)
The same number and kind of atoms are found on both sides.
Hydrogen + chlorine → hydrogen chloride
(hydrochloric acid)
H2 (g) + Cl2 (g)
→ HCl (g)
The equation does not accurately describe the reaction between
hydrogen and chlorine. Count the number of atoms present on both
sides of the equation.
An equation in which the reactants and products are not balanced
is sometimes called a “skeleton equation”. Atoms can not vanish;
therefore it is assumed that 2 molecules of HCl are created. To
show the both hydrogen chloride atoms in the product, a
coefficient “2” is placed before HCl (g) in the chemical equation.
H2 (g) + Cl2 (g)
→
2 HCl (g)
The coefficient applies to all the atoms in the molecule, 2
molecules of HCl (g). The chemical equation now follows the Law
of Conservation of Mass.
Figure 3: In (a) there are two (white) hydrogen atoms and two (green) chlorine atoms on
the left side of the scale, but only one hydrogen atoms and one chlorine atom on the right
side. In (b), there are two hydrogen atoms and two chlorine atoms on each side of the
scale. Reactants and products are balanced.
There is one molecule of methane and two molecules of oxygen
gas as the reactants. The products are two molecules of water and
one molecule of carbon dioxide, plus heat and light.
CH4 + 2O2 → 2H2O + CO2 + energy
If you count the number of each type of atom present in the
reactant portion of the equation and then compare that to the
number of each type of atom found in the products. You can see
that they are equal.
Therefore the total mass of the reactants is equal to the total mass
of the products, and the atoms of the reactants have been
rearranged to form products.
Check Your Learning, questions 1 – 7, pg. 232
6.4: Information in Chemical Equations
page 233
Skeleton Equation: is a representation of a chemical reaction
in which the formulas of the reactants are connected to the
formulas of the products by an arrow.
Word Equation:
Methane + oxygen → carbon dioxide + water
Skeleton Equation:
CH4 + O2 → CO2 + H2O
** The law of Conservation of Mass states that the mass of the
reactants equals the mass of the products.
There is an apparent imbalance between the numbers of atoms in
the reactants and the number of atoms in the products.
We can not change the types or formulas of the molecules. So how
can we solve this imbalance?
To do this, you need to change the number of molecules, not the
formulas.
Add one oxygen molecule to the reactants, and add a water
molecule to the products. The equation becomes balanced.
CH4 + O2 + O2 → CO2 + H2O + H2O
More appropriately, using coefficients, written as:
(usual way to write balanced equations is to use coefficients, which
indicate the number of atoms or molecules required)
CH4 + 2O2 → CO2 + 2H2O
How to Balance an Equation
Step 1.
Write the word equation for the reaction.
Iron + oxygen → magnetic iron oxide
Step 2.
Step 3.
Write the skeleton equation by replacing each name
with a correct formula.
Fe + O2 → Fe3O4
Count the number of atoms of each type in the reactants
and products.
**use a table like this:
Type of Atom
Fe
O
Step 4.
Reactants Products
1
2
3
4
Multiply each of the formulas by the appropriate
coefficient to balance the number of atoms.
Type of Atom
Reactants Products
Fe
O
3(1)
2(2)
3
4
The balanced chemical equation is written as this;
3Fe + 2O2 → Fe3O4
The formulas are unchanged, and the numbers of atoms are
balanced.
i.e., 2: Magnesium metal reacts with Nitric acid.
Step 1.
Write the word equation for the reaction.
magnesium + nitric acid → hydrogen + magnesium nitrate
Step 2.
Write the skeleton equation by replacing each name
with a correct formula.
Mg + HNO3 (aq) → H2 + Mg(NO3)2
Step 3.
Count the numbers of atoms of each type in the
reactants and products. This example is complicated by
the polyatomic nitrate ion.
Type of Atom
Mg
H
N
O
Reactants Products
1
1
1
3
1
2
2
6
Step 4:
Multiply each of the formulas by the appropriate
coefficient to balance the numbers of atoms.
Type of Atom
Mg
H
N
O
Reactants Products
1
2(1)
2(1)
2(3)
1
2
2
6
Mg + 2HNO3 (aq) → H2 + Mg(NO3)2
The equation is now balanced.
Check Your Learning: questions 1 – 8, pg. 236
6.5: Types of Chemical Reactions: Synthesis and
Decomposition
page 237
Chemist will know which elements and compounds undergo
particular types of chemical reactions. Elements are classified into
different chemical families. All members of a chemical family will
react a similar way. (valence electrons)
Compounds are also classified as ionic or molecular, and have
different patterns of chemical properties. Using these patterns,
chemical changes can be classified into groups.
These groups are; synthesis, decomposition, single and double
displacement reactions.
Knowing the types of reactions allows you to:
a) You can better understand experimental observations of the
behaviour of substances in chemical changes.
b) You can predict the products of unknown reactions.
Synthesis Reactions (combination reaction)
Synthesis reactions involve the combination of smaller atoms
and/or molecules into larger molecules. These reactions follow this
general formula:
A +
B
→
AB
Figure 2: In some cases, the reactants are atoms (elements), while in others, they are
molecules (elements or compounds)
These types of reactions can also involve combinations of small
molecules that form one larger molecule.
i.e., hydrogen chloride + ammonia → ammonium chloride
HCl + NH3 → NH4Cl
water + carbon dioxide → carbonic acid
CO2 + H2O → H2CO3 (aq)
Decomposition Reactions
Decomposition reactions involve the splitting of a large molecule
into elements of smaller molecules. These reactions follow this
general formula:
AB
→
A + B
Figure 3: In a decomposition reaction, a complex molecule breaks down, or decomposes,
into simpler products. The products can be elements or compounds.
Check Your Learning: questions 1 – 4, pg. 239
6.6: Types of Chemical Reactions: Single and Double
Displacement
page 240
What happens when an element and a compound combine in a
chemical reaction?
What happens when two ionic compounds react with each other?
In each type of displacement reaction, atoms are recombined to
form new compounds.
How do we decide which element is displaced? Your knowledge of
the periodic table and naming compounds is helpful. Metals found
on the left and centre of the periodic table. Nonmetals are found on
the right side of the table. The general rule is metal replaces metal
and nonmetal replace nonmetal.
Single Displacement Reactions
Single displacement reactions are chemical changes that involve an
element and a compound as reactants. One element displaces or
replaces another element from a compound. Single displacement
reactions have the following general formula:
Z+ + AB → ZB + A+
or
Y + AB → AY + BThis reaction can be represented as the following word, skeleton,
and chemical equations.
magnesium + silver nitrate → silver + magnesium nitrate
Mg + AgNO3
→ Ag
+ Mg(NO3)2
Mg + 2AgNO3 → 2Ag + Mg(NO3)2
Double Displacement Reactions
Double displacement reactions occur when elements in different
compounds displace each other or exchange places. Double
displacement reactions have the following general formula:
AB + ZY → AY + ZB
A and X are metallic elements and B and Y are nonmetallic
elements. During the reaction, B and Y (or A and X) exchange
places.
This reaction can be represented in the following word, skeleton,
and chemical equations.
lead (II) nitrate + potassium iodide →
lead (II) iodide + potassium nitrate
Pb(NO3)2 (aq) + KI (aq) → PbI2 (s) + KNO3 (aq)
Pb(NO3)2 (aq) + 2KI (aq) → PbI2 (s)
PbI2 (s)
+ 2KNO3 (aq)
is a yellow precipitate.
Precipitate: a solid formed from the reaction of two solutions.
Through experimentation, it was determined that some ionic
compounds do not dissolve in water. If insoluble compounds are
formed during the reaction and become visible, it is called a
precipitate. Not all double displacement reaction result in the
formation of a precipitate, but most do.
Check Your Learning: questions 1 – 8, pg. 243
6.9: Combustion
page 248
There are different categories of chemical reactions; combustion,
synthesis, decomposition, single and double displacement.
Combustion is a rapid reaction of a substance with oxygen, which
produces compounds called oxides, and releases energy in the form
of light and heat.
Complete Combustion
If the oxygen available is plentiful, hydrocarbons burn completely
to release the energy they contain. The only products of complete
combustion are carbon dioxide and water.
i.e.,
fuel + oxygen → oxides and energy
The fuels in which we commonly burn are Hydrocarbons.
(gasoline, natural gas, kerosene and candles)
Hydrocarbon – a compound composed of hydrogen and carbon.
The products from a combustion reaction are carbon dioxide and
water. Millions of tons of carbon dioxide and water are released
directly into the atmosphere.
i.e., hydrocarbon + oxygen → carbon dioxide + water
Word Equation:
butane + oxygen → carbon dioxide + water + energy
Balanced Chemical Equation:
2C4H10 + 13O2
→
8CO2 + 10H2O + energy
Incomplete Combustion
Incomplete combustion occurs when there is not enough oxygen
available. Instead of producing carbon dioxide and water, the
products released are carbon monoxide, carbon, carbon dioxide
and water.
Word Equation:
butane + oxygen → carbon + carbon + carbon + water + energy
monoxide
dioxide
Balanced Chemical Equation:
C4H10 + 5O2 → CO (g) + C(s) + 2CO2 (g) + 5H2O (g) + energy
Carbon monoxide is a colourless and odourless gas and it is a
very poisonous gas. Your cells die, because they are unable to
obtain oxygen. Hemoglobin which normally combines to oxygen,
to deliver it to your cells, prefers to combine with carbon
monoxide gas instead.
Check Your Learning: questions 1 – 7, pg. 251
6.10: Corrosion
page 252
Corrosion: the breakdown of a metal resulting from reactions with
chemicals in its environment.
Beneficial Corrosion
Sometimes corrosion of some metals forms a tough protective layer
that prevents further corrosion. For example,
4 Al(s) + 3 O2(g)
2 Al2O3(s) + energy
Causes of Rust
Rust
 Rusting is the corrosion or iron and steel that leads to the degradation
and damage of the metal. Rusting is caused or accelerated by
exposure to oxygen
exposure to water
explosure to electrolytes
Preventing Corrosion
Corrosion is the breakdown of a metal as it reacts with chemicals in the
environment.
Methods of preventing corrosion include
protective coatings (paint, plastic coatings, chrome)
corrosion-resistant materials (stainless steel, plastics)
galvanizing
Check Your Learning: questions 1 – 7, pg. 254
Chapter 6 Review: questions 1 – 18, pg. 258 - 259